The present invention relates to signal coding and in particular, though not necessarily, to the digital coding of speech signals.
In modern cellular radio telephone systems, sampled and digitised speech signals are coded prior to transmission over the air interface to reduce the channel bandwidth occupied by the transmitted signal. Considering for example the Global System for Mobile Communications (GSM) Phase 1 defined by the European Telecommunications Standards Institute (ETSI), speech signals are divided into time frames of 20 ms and each frame is coded using a Regular Pulse Excitationxe2x80x94Long Term Prediction (RPE-LTP) algorithm to remove long and short term redundancy from the signal. The result of this coding process is a set of 260 bits for each 20 ms frame.
It is also desirable to allow a receiver to identify and possibly correct errors introduced into the coded signal during transmission. This can be done for example by error correction coding. However, error correction coding introduces redundant information into the transmitted signal thereby increasing the signal bandwidth. To avoid increasing signal bandwidth by too great an extent, the GSM Phase 1 system provides error correction coding for only the 182 subjectively most important bits of a coded frame, leaving the remaining 78 bits unprotected. In particular, GSM uses a 3-bit cyclic redundancy check (CRC) for the 50 most important bits in addition to xc2xd-rate convolution coding for the 182 most significant bits (including the 50 bits protected by CRC). CRC and convolution coding increases the number of bits per frame from 260 to 456 (the 456 bit unit is often referred to as a xe2x80x9cchannel encoded framexe2x80x9d or merely as a xe2x80x9ccodewordxe2x80x9d).
In GSM, a codeword is transmitted by way of a series of radio bursts, with xe2x80x9cfrequency hoppingxe2x80x9d being used to shift the carrier frequency of a burst relative to that of the preceding burst and of the succeeding burst. This use of frequency hopping tends to reduce the effect of so-called xe2x80x9cburst errorsxe2x80x9d which disrupt data on a given carrier frequency. The effect of a burst error can also be spread more evenly across a codeword by interleaving the bits of the codeword in the radio bursts. This tends to increase the ability of the convolution coding to correct burst errors. The general concept of interleaving is illustrated in FIG. 1, where the 456 bits of a codeword are interleaved on four successive radio bursts, each burst containing 114 information bits (a burst typically contains other data bits but these are omitted from FIG. 1 in the interest of clarity).
GSM interleaves the bits of a codeword regardless of whether or not the bits are protected or unprotected. Consider for example the simplified case of a 40 bit codeword consisting of a stream of 25 protected bits A0 to A24 followed by a stream of 15 unprotected bits. The unprotected bits consist of three multi-bit coding parameters; X0 to X4, Y0 to Y6, and Z0 to Z2. The 40 bits are interleaved into four 10 bit radio bursts as illustrated in FIG. 2. As already discussed, successive bursts are transmitted on different carrier frequencies.
Consider the case where one of the radio bursts of FIG. 2 is totally corrupted by a burst error, leaving the other three bursts error free. Regardless of which burst is corrupted, one of the X and the Y parameters will be corrupted (1.0 error probability) whilst there is a 0.75 probability that the Z parameter will be corrupted. In GSM Phase 2 (which uses the Enhanced Full Rate speech codec: GSM 06.60), the parameters X, Y, and Z typically define the positions of respective pulses in an excitation vector. As such, all of the information conveyed by a parameter is lost if even only one parameter bit is erroneous. There is thus a high probability that a single burst error will result in the effective loss of the excitation vector.
As has been stated, FIG. 2 illustrates only a simplified example of interleaving. A more detailed description of the channel coding and interleaving processes used in GSM is given in xe2x80x9cThe GSM System for Mobile Communicationsxe2x80x9d by Mouly and Pautet, 1992, ISBN: 2-9507190-0-7. ETSI recommendation GSM 05.03 November 1997 provides details of channel coding and interleaving processes proposed for GSM Phase 2+. It is noted in particular that in this GSM recommendation (Chapter 3.1.3; Table 1) codeword bits are inserted diagonally into the burst structure, rather than in the vertical columns illustrated in FIGS. 1 and 2.
It is an object of the present invention to overcome or at least mitigate the disadvantage of conventional interleaving processes noted in the preceding paragraph. In particular, it is an object of the present invention to provide for the interleaving of codeword bits in such way that unprotected multi-bit parameters are made more robust against burst errors.
These and other objects are achieved by interleaving unprotected bits in a parameter-wise fashion, rather than in a bit-wise fashion.
According to a first aspect of the present invention there is provided a method of transmitting a codeword over a transmission channel using a plurality of radio bursts, the codeword comprising a first ordered sequence of protected bits and a second ordered sequence of unprotected bits, and the radio bursts together providing a set of chronologically ordered bit positions, the method comprising:
defining first and second sets of bit positions in the radio bursts;
allocating successive bits of said first sequence to said first set of bit positions in a cyclical manner with respect to the radio bursts so that adjacent protected bits are allocated to different radio bursts;
allocating successive bits of said second sequence to said second set of bit positions in the chronological order of those bit positions; and
transmitting the radio bursts comprising the allocated bits.
Embodiments of the present invention provide for the interleaving of protected bits of a codeword amongst radio bursts so that the coding process by which these bits are protected may be robust against transmission errors, such as burst errors. These embodiments also achieve the approximate grouping of parameter bits, making up the unprotected parameters, into a single or limited number of radio bursts, thus minimising the number of parameter which will be affected by a single burst error.
The codeword may comprise additional unprotected bits which are allocated to the radio bursts together with the protected bits of said first sequence in said cyclical manner. This may be appropriate for example where a coding parameter consists of a number of codeword bits, only one of which is unprotected such that there is no benefit to be achieved by including that unprotected bit in said second sequence.
Preferably, said plurality of radio bursts are transmitted sequentially and on different frequency bands. A radio burst may additionally contain data bits other than those allocated from the codeword.
Preferably, the codeword comprises coded data corresponding to a time frame of an audio signal. More preferably, the audio signal is a speech signal. The method of the present invention is particularly applicable to cellular radio telephone systems where the transmission channel is the air interface between a mobile communications device and a base transceiver station of the cellular system.
Preferably said second ordered sequence comprises a plurality of multi-bit coding parameters and the bits of a given parameter occupy neighbouring positions in the sequence.
Preferably, said first ordered sequence of bits is protected by convolution coding. The first sequence may additionally be protected by a cyclic redundancy check code.
Embodiments of the present invention may be arranged to transmit at least two codewords using said plurality of radio bursts, the method comprising interleaving the bits of the codewords in the bit positions of the radio bursts. For example, for a first codeword, said first and second sets of bit positions may be provided by even numbered bit positions of the radio bursts, whilst for a second codeword said first and second sets of bit positions may be provided by odd numbered bit positions. The radio bursts may comprise additional data bits such as those making up a training or synchronisation sequence for use by the receiver.
According to a second aspect of the present invention there is provided apparatus for transmitting a codeword over a transmission channel using a plurality of radio bursts, the codeword comprising a first ordered sequence of protected bits and a second ordered sequence of unprotected bits, and the radio bursts together providing a set of chronologically ordered bit positions, the apparatus comprising:
an input for receiving the codeword;
a memory for defining first and second sets of bit positions in the radio bursts;
signal processing means for allocating successive bits of said first sequence to said first set of bit positions in a cyclical manner with respect to the radio bursts so that adjacent protected bits are allocated to different radio bursts, and for allocating successive bits of said second sequence to said second set of bit positions in the chronological order of those bit positions; and
a transmitter for transmitting the radio bursts comprising the allocated bits.
Preferably, the apparatus of the present invention comprises a speech encoder for encoding time frames of an audio signal into respective codewords, and for providing the codewords to said input.
According to a third aspect of the present invention there is provided a mobile communications device comprising apparatus according to the above second aspect of the invention.
According to a fifth aspect of the present invention there is provided a base station controller of a cellular radio telephone network, the base station controller comprising apparatus according to the above second aspect of the present invention.